Many people have encoutnered issues with the TOE on the Dell PowerEdge servers (especially the 2900) where it caused weird issues with networking components, most noticeable on VMware boxes and ISA servers.
A few days ago I had the same issue with our PowerEdge 2900 after I upgraded to the latest drivers and firmware, the server was running windows 2003 with SP2 loaded on it, once the server was rebooted, the host OS could not access the guest systems via UNC or RDP ( Remote Desktop) however I was able to ping the guest OS by IP and common name with no issues.
after researching the issue, I discovered, it is a problem with the Broadcom Network Cards, Windows 2003SP2 and the TOE (Tcpip Offload Engine)
I tired many different thigs except uninstalling the SP2 from the Host OS as it was not a fix for me (not good enough solution) so what I did was opened up the case and removed the TOE component which is like a little white adapter that plugs into the motherborad of the 2900 PowerEdge server. The connector itself is like a phone plug. I have attached a picture of the adapter.
To remove the TOE adapter, slide open the case, look in the center next to the CPU, there will be a white chip (looks like a one of those tranceievers) sticking out and labeled TOE 2) Just unplug it (you would unplug it just like how you unplug a phone from a phone jack).
Once I removed the TOE adapter from the server and powered it back on, everything worked like a charm. I phoned Dell and talked to their senior techs and they did elaborate that this was a problem they have been running into recenlty and this was the right fix.
I hope this helps you out. if it does please post to let me know.